Evaluating Component Substitution Costs in High-Density Power Electronics
Evaluating power component substitutions requires amortizing NRE and re-qualification expenses against true volume savings while accounting for parasitic and thermal shifts.
Miller Capacitance
Meaning
Parasitic semiconductor properties represent the internal feedback capacitance between the drain and the gate terminals of a field effect transistor. This parameter, often called Miller Capacitance, slows down the switching transition by requiring additional charge to change the voltage on this internal node. It must be managed to prevent slow turn-on times and elevated power losses.
Gate Charge
Charging the internal feedback path requires a specific portion of the gate current during the switching transition. This effect of Miller Capacitance is visible as a flat region in the gate charge curve where the voltage remains constant. The gate driver must supply sufficient current to quickly move through this region.
Switch Interlock
Rapid voltage transitions can cause the parasitic capacitance to inject current into the gate of an off-state switch. If this occurs, Miller Capacitance can turn the device back on, causing a destructive short circuit in bridge configurations. This risk necessitates the use of gate drivers with high pull-down strength.
Module Cost
Selecting transistors with low feedback capacitance is an important decision in the development of high-frequency power converters. Although components with optimized Miller Capacitance command a premium price in supply contracts, they allow the use of smaller and cheaper passive components elsewhere in the circuit. This choice directly affects both the production cost of the module and the design of the cooling system.
This optimization determines the competitive positioning of the converter in the market.